Into the unknown: Novel method for screening water for unknown contaminants

Ezine

Published: Dec 11, 2017

Author: Jon Evans

Channels: Ion Chromatography

Toxic contaminants

The problem with monitoring drinking water for potentially toxic contaminants is that there are just so many to test for, coming from industrial processes, pesticides, general environment pollutants and even by-products from the chemicals used to disinfect water. Moreover, as these processes, pesticides, pollutants and disinfection chemicals are changing all the time, so are the contaminants that need to be tested for.

Now, however, Leon Barron and his colleagues at King’s College, London, in the UK have come up with a neat little solution to this problem. Their idea is to produce monitoring data that is sufficiently accurate and detailed that not only can it be used to identify known contaminants, but can also be screened for the presence of formerly unknown contaminants. To produce this sufficiently detailed monitoring data, they turned to a combination of ion-exchange chromatography (IEC) and high-resolution mass spectrometry (HRMS).

As a first test of this idea, they decided to search for haloacetic acids (HAAs), a type of halogenated carboxylic acid that is a known disinfection by-product. Because HAAs pose long-term health risks for humans, regulatory authorities set strict thresholds for allowable concentrations in drinking water, which means those concentrations require regular monitoring.

Orbitrap MS

Barron and his colleagues began by developing an optimized IEC-HRMS method for detecting HAAs, using an Exactive Orbitrap from Thermo Fisher Scientific as their HRMS system. To enhance the sensitivity of their method as much as possible, they also added a solid-phase extraction step. After testing four hydrophobic sorbent materials, they found that one called Isolute ENV+ proved best at extracting and concentrating HAAs. A further advantage of this sorbent is that it can extract quite a wide range of organic compounds from water, which should prove handy when searching for unknown contaminants.

They then tested this method on water spiked with 10 HAAs, which they detected by searching the mass spectra for the same three characteristic fragment ions for each HAA. One great advantage of the Orbitrap is that it could detect the masses of these fragments so accurately that they could be distinguished from each other even if they eluted at similar times. It could also distinguish them from other potentially interfering compounds, such as chlorides, nitrates and sulfates.

The other great advantage of the Orbitrap is that it meant the HAAs could be identified in three separate ways: from their retention times, from the accurate measurement of the mass of the fragments, and by using this mass measurement to determine the ratio of different isotopes in the fragments. As a result, the scientists could be confident in any identification, which again should prove handy when searching for unknown contaminants.

Known unknowns

Using this approach, Barron and his colleagues found they could detect the 10 HAAs in water at low microgram per liter concentrations, which is sufficiently sensitive for regulatory monitoring. So they next tried analyzing unspiked drinking water, comparing their novel IEC-HRMS method with IEC combined with conductivity detection. Whereas they were unable to detect any HAAs using IEC with conductivity detection, they could detect all 10 with their optimized IEC-HRMS method, albeit at concentrations below the regulatory thresholds.

Finally, they analyzed the spectral data from the unspiked water for evidence of 15 other halogenated carboxylic acids, for which the method hadn’t been optimized. Once again, they looked for the same three characteristic fragment ions for each halogenated carboxylic acid, and again utilized a combination of the retention time, the mass of the fragments and the isotope ratios for identification.

In this way, they were confidently able to identify four halogenated carboxylic acids in the water: two chlorinated propionic acids, monochloropropionic acid (MCPA) and dichloropropionic acid (DCPA), and two iodinated HAAs, monoiodoacetic acid and chloroiodoacetic acid. Barron and his colleagues suggest that DCPA likely comes from the herbicide Dalapon, while MCPA had never been detected in drinking water before.

So with this one method, analytical scientists can now find contaminants they knew they were looking for and also contaminants they didn’t know they were looking for.